As shown in FIG. 1, a rolling bearing is composed of a concentrically assembled set of inner race 2 forming inner raceway track 1 on its outer circumference and outer race 4 forming outer raceway track 3 on its inner circumference, with a plurality of rolling members 5 rotatably arranged between inner raceway track 1 and outer raceway track 3. Outer edge of a doughnut-shaped seal plate 6 is fixed to the inner surface of outer race 4 on both sides thereof so that the grease applied around rolling members 5 or dust generated may not leak outside or dust suspending outside may not enter in the vicinities of the rolling members 5.
Plural rolling members 5 are rotatably held by cage 7 shown in FIG. 2. Cage 7 of this type is made of an integrally injection-molded synthetic resin. Cage 7 integrally consists of annular main body 8 having a plurality of holding parts 9 on one side thereof. Each holding part 9 is formed by a pair of elastic parts 10 spaced from each other. The opposing faces of a pair of elastic parts 10 have a concave curved surface, thereby making a spherical form or a column form. Each rolling member 5 is pressed into the space between a pair of elastic parts 10 while elastically widening the interval between the pair of elastic parts 10 and is thereby rotatably held by each holding part 9.
Cage 7 is generally made of a metal or a synthetic resin which can be injection molded, such as a polyamide resin, a polyacetal resin or a polybutylene terephthalate resin, or a synthetic resin composition comprising such a synthetic resin and glass fiber, carbon fiber, organic fiber, etc.
Since cage 7 only functions to hold rolling members 5, lubrication of a rolling bearing is achieved by using a large amount of a lubricating oil or a semi-solid lubricant, such as grease. However, when, in particular, a semi-solid lubricant such as grease is used in case where a large amount of a lubricating oil cannot be used, resistance to stirring of the lubricant increases the torque required for rotating a shaft, etc. supported by the bearing. Besides, the torque fluctuation increases with rotation.
In order to overcome these problems, a rolling bearing in which cage 7 itself has a lubricating function has been proposed. For example, JP-A-61-6429 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a bearing cage comprising a porous polyamide-imide resin formed by compression molding and impregnated with a fluorinated oil. JP-A-1-93623 discloses a cage which is produced by injection molding an oil-containing plastic composition comprising a matrix resin and an oil-containing binder to obtain a cage precursor and then impregnating the cage precursor with a lubricating oil by immersion.
Since the lubricating oil incorporated into the cage of these rolling bearings slowly oozes from the surface of the cage with rotation of the bearing, the bearing exerts lubricating action for a long time. However, the conventional rolling bearings do not always satisfy the demands for lubrication in the initial stage of rotation or at a low speed of rotation.
Further, the cage of JP-A-61-6429 oozes the lubricating oil soon due to its porosity and loses the lubricating action in the relatively early stage of use.
On the other hand, as in JP-A-1-93623, when an oil-containing plastic composition having an oil content of about 10% by weight is injection molded into such a shape as a bearing cage that is apt to cause weld lines, the lubricating oil tends to be separated from the matrix resin while the composition is flowing in the mold, and the resulting molded article suffers from considerable reduction in mechanical characteristics in the vicinities of the weld lines.
Besides, taking affinity to the resin matrix into consideration, it is difficult in the above techniques to use a lubricating oil containing additives, such as a rust inhibitor, for the purpose of imparting not only lubricity but other properties, such as anticorrosion, to the rolling bearing.